A direct oxidation fuel cell (DOFC) is an electrochemical device that generates electricity from electro-oxidation of a liquid fuel. Liquid fuels of interest include methanol, formic acid, dimethyl ether (DME), etc. and their aqueous solutions. The oxidant may be substantially pure oxygen or a dilute stream of oxygen such as that in air. Significant advantages of employing a DOFC in portable and mobile applications (e.g. notebook computers, mobile phones, PDAs, etc.) include easy storage/handling and high energy density of the liquid fuel.
One example of a DOFC system is a direct methanol fuel cell or DMFC. A DMFC generally employs a membrane-electrode assembly (hereinafter “MEA”) having an anode, a cathode, and a proton-conducting membrane electrolyte put therebetween. A typical example of the membrane electrolyte is one composed of a perfluorosulfonic acid-tetrafluorethylene copolymer such as NAFION, NAFION is a registered trademark of E. I. Dupont de Nemours and Company). In a DMFC, a methanol/water solution is directly supplied to the anode as the fuel and air is supplied to the cathode as the oxidant. On the anode, methanol reacts with water in the presence of a catalyst, typically a Pt or Ru metal-based catalyst, to produce carbon dioxide, protons and electrons. The electrochemical reaction is shown as equation (1) below.CH3OH+H2O→CO2+6H++6e−  (1)
During operation, the protons migrate to the cathode through the proton-conducting membrane electrolyte, which is non-conductive to electrons. The electrons travel to the cathode through an external circuit where electric power is delivered. On the cathode, the protons, electrons and oxygen molecules, typically from air, are combined to form water. The electrochemical reaction is given in equation (2).3/2O2+6H++6e−→3H2O  (2)These two electrochemical reactions form an overall cell reaction as shown in equation (3).CH3OH+3/2O2→CO2+2H2O  (3)
One drawback of a conventional DMFC is that the methanol partly permeates the membrane electrolyte from the anode to the cathode, such permeated methanol is called “crossover methanol”. The crossover methanol reacts with oxygen at the cathode, causing reduction in fuel utilization efficiency and cathode potential so that power generation of the fuel cell is suppressed. It is thus conventional for DMFC systems to use excessively dilute (3-6% by vol.) methanol solutions in the anode in order to limit methanol crossover and its detrimental consequences. However, the problem with such a DMFC system is that it requires a significant amount of water to be carried in a portable system, thus sacrificing the system energy density.
The ability to use high concentration fuel is highly desirable for portable power sources particularly since DMFC technology is competing with advanced batteries such as lithium-ion technology. However, even if the fuel cartridge carries little to no water, the anodic reaction, equation (1), still needs one water molecule per methanol molecule for complete electro-oxidation. Conversely, water is produced in the cathode from the reduction of oxygen, equation (2). Therefore, to take full advantage of a fuel cell employing high concentration fuel it would be desirable: (1) to maintain a net water balance in the cell where the total water loss from the cell (mainly through the cathode) should preferably not exceed the net production of water (i.e. two water molecules per every methanol molecule consumed according to equation (3)), and (2) to transport some of the produced water from the cathode to anode.
Two approaches have been developed to meet the above-mentioned goals in order to directly use concentrated fuel. One is an active water condensing and pumping system to recover cathode water vapor and return it to the anode (U.S. Pat. No. 5,599,638). While this method achieves the goal of carrying concentrated and even neat methanol in the fuel cartridge, it suffers from a significant increase in system volume and parasitic power loss due to the need for a bulky condenser and its cooling/pumping accessories.
The second approach is a passive water return technique in which the hydraulic pressure in the cathode is built up by applying a highly hydrophobic microporous layer (MPL) in the cathode and this pressure drives water from the cathode to the anode through a thin membrane (Ren et al. and Pasaogullari & Wang 2004). While this passive approach is efficient and does not incur parasitic power, the amount of water returned and hence the concentration of methanol fuel depends strongly on the cell temperature and power density. Presently, direct use of neat methanol is demonstrated only at or below 40° C. and at low power (less than 30 mW/cm2). Considerably less concentrated methanol fuel is in high-power (e.g. 60 mW/cm2) systems at elevated temperatures such as at 60° C. In addition, the need for thin membranes in this method sacrifices fuel efficiency and operating cell voltage, thus resulting in lower total energy efficiency.
There is thus a prevailing need to provide a direct oxidation fuel cell system that automatically maintains a balance of water in the fuel cell and returns a sufficient amount of water from the cathode to the anode under high-power and elevated temperature operating conditions. There is an additional need to provide a direct oxidation fuel cell that operates directly on highly concentrated fuel, including neat methanol, and minimizes the need for an external water supply or the condensation of electrochemically produced water.